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D5 further cosmology


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cosmology advanced

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D5 further cosmology

  1. 1. Further Cosmology D5
  2. 2. Historically, “cosmology” was the realm of philosophers.
  3. 3. Isaac Newton Thought that an isolated group of stars would collapse in on itself. An infinite universe of stars should collapse into isolated islands of mass. A finely tuned universe could be balanced and static.
  4. 4. Albert Einstein General Relativity: viewing gravity as curved space time (1915). Einstein thought the universe was static and unchanging, although his equations were dynamic. Added a cosmological constant term which acts as an repulsive force, balancing gravity.
  5. 5. Alexander Friedmann He came to the conclusion that Einstein’s cosmological equations predicted that the universe evolved with time, either expanding or collapsing. Einstein wrote that Friedmann had made a mathematical error and his results were invalid.In 1923, Einstein retracted his objection and agreed relativistic universe was dynamic.
  6. 6. Einstein’s Biggest Blunder? After Friedmann’s work, Einstein threw away his Cosmological Constant, calling it his biggest blunder. The addition of a cosmological constant term was a completely legitimate mathematical exercise. Einstein’s blunder was choosing a specific value for the cosmological constant to balance gravity, not its addition. It was not discarded, just set to zero.
  7. 7. Edwin Hubble In the 1920s, Hubble measured the speeds of nearby galaxies. He found nearly all were rushing away from us, with their velocity increasing with distance, exactly as predicted in the
  8. 8. Cosmological Principle The distribution of matter across the Universe is approximately even, homogeneous, when considered at large scales.
  9. 9.
  10. 10. Early universe Planck time (<10-43s) Quarks, leptons, particle and anti-particle Protons and neutrons form/annihilation results in photons Expansion results in stretching of photons and not so many hadrons produced Nuclei form Expansion and cooling
  11. 11. Understanding Expansion As we go back in time, the scale factor R(t) goes to zero. This means the distance between any two objects also goes to zero. This is the location of the “Big Bang”.
  12. 12. Which Scale Factor? The shape of the scale factor depends upon the mix of energies in the universe. Universes only containing matter slow down over time, while other universes slow and then accelerate. Which is our universe?
  13. 13. Possible universes If mass a curves space-time then the universe will have curves. Omega =0 is flat universe Omega <1 is open universe Omega > 1 is closed universe
  14. 14. Friedmann Universes These open, flat, closed scenarios are known as Friedmann universes.
  15. 15. The fate? The fate of the universe depends on its density. If we consider the universe as a sphere of radius R then the matter has both potential and kinetic energy PE + KE = -GMm/R + 1/2mv2 Can you show that the critical density (to give a flat universe) is 3v2/8πGR2?
  16. 16. The Future of the Universe The mix of matter and energy imply that the expansion of the universe is beginning to accelerate, and in the future, the universe will dilute and dim.
  17. 17. Georges Lemaître Friedmann died soon after he published his work. Georges Lemaitre examined the equations of cosmology, especially the point where the scale factor goes to zero. Running the universe backwards, he realized that it must have been hotter in the past. He proposed the “Hot Big Bang” model of the universe, where the universe was born in a hot, dense state and has been cooling and expanding ever since.
  18. 18. Cosmic Microwave Background Penzias & Wilson won the 1978 Nobel Prize for detecting the cosmic microwave background radiation. Mather & Smoot won the 2006 Nobel prize for showing this radiation has a blackbody spectrum (2.7K) and for revealing that it is not smoothly distributed over the sky.
  19. 19. Cosmic Microwave Background While the mean temperature of the sky is 2.7K, some regions are hotter and some cooler, with a temperature difference of 0.001K. Where did these temperature differences come from?
  20. 20. Anisotropies in CMB •
  21. 21. Cosmological Supernovae Supernovae are exploding stars whose true brightness is well known. Using the Keck and Hubble Space Telescope, we found supernovae appeared fainter than expected, showing that the universe does not contain only matter.
  22. 22. Which Universe? A third of the cosmos is matter, the most of which is dark (does not radiate, but we can feel its gravitational pull). Heavy elements (that’s us!) make up 0.03% of the universe. Some mysterious substance, dark energy, make up 70% of the universe.
  23. 23. Rotation curves Orbital speed versus orbital radius Applying newton’s laws V α 1/√r
  24. 24. Dark Matter Everywhere we look in the cosmos we see the gravitational influence of dark matter, from the rotation curves of galaxies, large scale motions, gravitational lenses, hot gas in clusters to the evolution of the entire universe. Is it a physical substance or physical fudge?
  25. 25. Dark Matter It has been proposed that dark matter can decay into normal matter, with an equal mix of matter and anti-matter. The PAMELA ( space-craft has been searching for the signal of this additional flux of anti-matter.
  26. 26. What is dark energy? Well, we know what it isn’t. It isn’t dark or normal matter. It has to possess a negative pressure (a tension) to cause the universe to accelerate. With quantum physics, the vacuum is not empty but seethes with particles popping in and out of existence. Such a vacuum possesses precisely the tension of dark energy Only problem is that the density is wrong by a factor of
  27. 27. CMB Hot & Cool Spots At the time of reionization (when the universe became neutral) there were regions of slightly higher and slightly lower density. Where did these come from?
  28. 28. How Big is the Universe? Given our mix of dark energy and matter, the universe is infinite in extent, but we can only see the “observable universe”. As time proceeds, more and more is revealed.
  29. 29. Possibilities? Reproducing Universe: Linde Some slides taken from a presentation by Prof Geraint Lewis Sydney Institute for Astronomy,University of Sydney